The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. Even though John Herschel is perhaps the inventor of the cyanotype process, Anna Atkins actually brought this to photography. She created a limited series of cyanotype books that documented ferns and other plant life. By using this process, Anna Atkins is regarded as the first female photographer.

They result in a photo-sensitive solution when dissolved in water, which is used to coat a material (usually paper). A positive image can be produced by exposing it to a source of ultraviolet light (such as sunlight) with a negative. The UV light reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue.

The developing of the picture takes place by flushing it with flowing water. The water-soluble iron(III) salts are washed away, while the non-water-soluble Prussian blue remains in the paper. This is what gives the picture its typical blue color. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, referred to as blueprints.

The process

In a typical procedure, equal volumes of an 8% (w/v) solution of potassium ferricyanide and a 20% solution of ferric ammonium citrate are mixed. This mildly photosensitive solution is then applied to a receptive surface (such as paper) and allowed to dry in a dark place. Cyanotypes can be printed on any surface capable of soaking up the iron solution. Although watercolor paper is a preferred medium, cotton, wool and even gelatinsizing on nonporous surfaces have been used.

Upon exposure to ultraviolet light (such as that in sunlight), the iron in the exposed areas will reduce, turning the paper a steel-grey-blue color. The extent of color change is dependent on the amount of UV light, but acceptable results are usually obtained after 10-20 minute exposures on a bright, sunny day. Prints can be made with large format negatives and lithography film, or everyday objects can be used to make photograms.

After exposure, the yellow, unreacted iron solution is rinsed off with running water. Although the blue color darkens upon drying, the effect can be accelerated by soaking the print in a 6% (v/v) solution of 3% (household) hydrogen peroxide.

Long-term preservation

In contrast to most historical and present-day processes, cyanotype prints do not like basic environments. So it is not a good idea to store or present the print in chemically buffered museum board. This will cause the image to fade. Another unusual characteristic of the cyanotype is its regenerative behaviour: prints that have faded due to prolonged exposure to light can often be significantly restored to their original tone by simply temporarily storing them in a dark environment.